Wireless Audio Networking Modifying the IEEE 802.11 standard to handle multi-channel, real-time wireless audio networks A thesis submitted for the degree of Doctor of Philosophy by Christos Chousidis College of Engineering, Design and Physical Sciences Brunel University August 2014 Abstract Audio networking is a rapidly increasing field which introduces new exiting possibilities for the professional audio industry. When well established, it will drastically change the way live sound systems will be designed, built and used. Today's networks have enough bandwidth that enables them to transfer hundreds of high quality audio channels, replacing analogue cables and intricate installations of conventional analogue audio systems. Currently there are many systems in the market that distribute audio over networks for live music and studio applications, but this technology is not yet widespread. The reasons that audio networks are not as popular as it was expected are mainly the lack of interoperability between different vendors and still, the need of a wired network infrastructure. Therefore, the development of a wireless digital audio networking system based on the existing widespread wireless technology is a major research challenge. However, the ΙΕΕΕ 802.11 standard, which is the primary wireless networking technology today, appears to be unable to handle this type of application despite the large bandwidth available. Apart from the well-known drawbacks of interference and security, encountered in all wireless data transmission systems, the way that ΙΕΕΕ 802.11 arbitrates the wireless channel access causes significantly high collision rate, low throughput and long overall delay. The aim of this research was to identify the causes that impede this technology to support real time wireless audio networks and to propose possible solutions. Initially the standard was tested thoroughly using a data traffic model which emulates a multi-channel real time audio environment. Broadcasting was found to be the optimal communication method, in order to satisfy the intolerance of live audio, when it comes to delay. The results were analysed and the drawback was identified in the hereditary weakness of the IEEE 802.11 standard to manage broadcasting, from multiple sources in the same network. To resolve this, a series of modifications was proposed for the Medium Access Control algorithm of the standard. First, the extended use of the "CTS-to-Self" control message was introduced in order to act as a protection mechanism in broadcasting, similar to the RTC/CTS protection mechanism, already used in unicast transmission. Then, an alternative "random backoff" method was proposed taking into account the characteristics of live audio wireless networks. For this method a novel "Exclusive Backoff Number Allocation" (EBNA) algorithm was designed aiming to minimize collisions. The results showed that significant improvement in throughput can be achieved using the above modifications but further improvement was needed, when it comes to delay, in order to reach the internationally accepted standards for real time audio delivery. Thus, a traffic adaptive version of the EBNA algorithm was designed. This algorithm monitors the traffic in the network, calculates the probability of collision and accordingly switches between classic IEEE 802.11 MAC and EBNA which is applied only between active stations, rather than to all stations in the network. All amendments were designed to operate as an alternative mode of the existing technology rather as an independent proprietary system. For this reason interoperability with classic IEEE 802.11 was also tested and analysed at the last part of this research. The results showed that the IEEE 802.11 standard, suitably modified, is able to support multiple broadcasting transmission and therefore it can be the platform upon which, the future wireless audio networks will be developed. i Contents Abstract . i Table of contents . ii List of figures . vi List of tables . viii Abbreviations . ix Acknowledgment . xii Author’s declaration . xiii Chapter 1: Introduction . 1 1.1 Overview. 2 1.2 Motivation . 2 1.3 Scope of the thesis . 3 1.4 Contribution to knowledge . 4 1.5 Publications arising from this research . 6 1.6 Thesis structure . 7 Chapter 2 : Background in Audio Networking (Introduction to Sound Systems, Fundamental of Audio Networks, Wireless Audio Networking and Literature Review) . 9 2.1 Introduction . 10 2.2 Fundamentals of Conventional Analogue Audio Systems . 10 2.3 Introduction to Audio Networking . 14 2.4 Existing Networking Audio Systems in the market . 14 2.4.1 Overview . 14 2.4.2 EtherSound by Digigram . 15 2.4.3 CobraNet by Cirrus Logic . 16 2.4.4 Aviom A-NET Pro16 & Pro64 . 16 2.4.5 Audiorail . 17 2.4.6 MaGIC by Gibson . 17 2.4.7 Dante . 18 2.4.8 HyperMAC & SuperMAC . 18 2.4.9 Livewire . 19 2.5 An overview of digital audio transfer standards . 20 2.5.1 AES-3 . 20 2.5.2 AES-10 (MADI) . 20 2.5.3 AES-47 . 21 ii 2.5.4 AES-50 (HRMAI) . 21 2.5.5 AES-67 (Standard for audio applications of networks - High- performance streaming audio-over-IP interoperability) . 22 2.5.6 EBU Doc 3326-2007 . 23 2.6 Drawbacks and problems in the deployment of audio networking . 23 2.7 Wireless Audio Networking . 24 2.8 Literature review on wireless audio networking . 25 2.9 Characteristics and demands of a Wireless Audio Network . 28 2.10 The IEEE 802.11 medium access control mechanisms . 30 2.10.1 General Description . 30 2.10.2 Analysis of the IEEE 802.11 MAC algorithms . 31 2.10.3 Drawbacks of Random Backoff in Wireless Broadcasting . 33 2.11 Literature review in reliable broadcasting in wireless Ad-Hoc networks . 34 2.11.1 Simple Flooding Method . 34 2.11.2 Probability Based Method . 34 2.11.3 Counter-Based Method . 35 2.11.4 Area-Based Method . 35 2.11.5 Neighbour Knowledge method . 35 2.11.6 Cluster Based methods . 36 2.11.7 State of the art in broadcasting in wireless ad-hoc networks . 36 2.12 Shortcoming of the existing broadcasting methods to handle real-time audio data . 37 2.13 Network modelling and simulation . 37 2.14 An overview of network simulation tools . 38 2.15 Discrete-event Simulation (DES) . 39 2.16 OPNET’s general characteristics . 40 2.16.1 Network Domain . 41 2.16.2 Node Domain . 41 2.16.3 Process Domain . 42 2.16.4 Finite State Machines . 42 2.16.5 OPNET process modelling . 43 2.16.6 OPNET project and project editor . 44 2.17 Summary . 44 Chapter 3: Expanding the use of CTS-to-Self mechanism to improve broadcasting on IEEE 802.11 networks . 46 3.1 Introduction . 47 3.2 RTS/CTS protection mechanism . 47 3.3 The regular use of CTS-to-Self control message in IEEE 802.11 . 49 iii 3.4 Probability of collision in a multi-broadcasting wireless network . 51 3.4.1 Calculating the probability of collision in broadcasting in.